The present invention relates to the bonding of electrical wires to a terminal and particularly to the bonding of insulated copper armature winding wires to commutators.
In the modern manufacture of motor armatures, the rapid winding of the insulated magnet wire onto the motor armature is an economic necessity. To achieve high winding speeds, the magnetic wire insulation must exhibit a high degree of erosion resistance for the insulation to survive the mechanical stresses involved.
Modern day high performance magnetic wire insulation achieves the required erosion resistance by the addition of air and moisture sensitive organo-titanium compounds mixed in with the insulation resin chemistry. Small amounts of such titanium compounds (i.e., a few hundred ppm) have been used by formulators for a long time to catalyze various organic reactions occurring during the formulation of their resins. Once these compounding chemistries are complete, and on exposure to air and moisture, the organo-titanium additions produce very fine dispersions of hydrated titanium oxides. It was then recognized that these dispersions improved the resistance of the magnetic wire to the erosive forces which occur during the winding operation. Over the years, the titanium concentrations in the resin have been increasing. These titanium levels are generally on the level of 0.3 to 0.8 weight percent titanium calculated as the metal.
The magnetic wire of the armature is attached to the commutator of the armature by means of connectors which are referred to as risers or tines. These tines are hooked-shaped tabs connected to each commutator segment through which the end of the each magnetic wire passes. The tine is crimped over the wire and an electrical current is then passed through the tine to spot weld or fuse the wire to the tine. During the passage of electrical current, the organic resin component of the insulation is burned, converting the organic resin component to gaseous oxides. However, the comparatively refractory inorganic component (i.e., titanium oxides) remains. The titanium oxides that are now present at the interface between the tine and the magnetic wire prevent the formation of a metallurgical bond between the two components. A relatively high resistance barrier is formed across the joint. At most, there are only isolated points of direct contact between the wire and the commutator tine. It is basically only a mechanical bond with the wire and tine merely in physical contact with each other through an intervening insulating oxide layer.
In the short term, productivity is diminished because of these poor bonds since a small fraction of the motors produced will have unacceptably high electrical resistances. In the long term, some motors which are initially acceptable will fail because electrical resistances can increase during service. This happens because the titanium oxides may not be in the stable high temperature forms, i.e., rutile, anatase or brookite. Instead, they may be in the hydrated, lower temperature forms. These hydrated forms will release water when heated during operation and can also pick up water from the ambient environment when cold. These transformations will result in volume changes in the oxide producing fatigue effects in the bond. Also, since the hydrated oxide layer does not form a hermetic seal at the interface, atmospheric moisture and other corrodents can have easy access to the bond line during the service life. It can be seen that the inclusion of the organotitanium compounds in the insulation resin chemistry creates both a benefit and a problem.